A problem that is consistently encountered with the use of aromatic polyester polyols in the manufacture of rigid polyurethane and urethane-modified polyisocyanurate foams is compatibility with the trichlorofluoromethane blowing agent commonly used in foam formulations. The two methods commonly used to overcome this problem are (1) incorporation of polypropylene glycol into the polyol molecular structure, either by propoxylation or addition of dipropylene glycol, and (2) the addition of a compatible polyether polyol or a compatibilizer such as an alkyl phenol ethoxylate. Although both methods work with varying degrees of success with some aromatic polyester polyols, neither is completely successful when the polyols are derived from dimethyl terephthalate (DMT) process residue.
If dipropylene glycol is incorporated into the molecular structure of an aromatic polyester polyol derived from DMT process residue, the final product viscosity is too high for convenient processing of the product. To achieve a viscosity of less than 20,000 cps at 25.degree. C, the amount of free glycol remaining in the product has to be increased, thus increasing the product hydroxyl number, i.e., reducing the equivalent weight. The current emphasis in the rigid foam market is on higher isocyanate to hydroxyl ratios. Low equivalent weight polyols are therefore not desirable, since the amount of isocyanate needed to achieve the required ratio, and therefore the cost, would have to be increased. In addition, producers of rigid foam products have increased the level of aromatic polyester polyols in their formulations. Because of this increase in the aromatic content of rigid foams, polyester polyols derived from DMT process residue, which are highly viscous and difficult to process, are unsuitable for use in these formulations.